The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Recently, there has been an increasing need to permit the quick, non-invasive, non-destructive analysis of the chemical compositions of materials, especially when such materials are concealed, such as illicit materials or explosives, or otherwise spaced a considerable distance from the detection system, such as to improve personnel safety in the event of an explosion. That is, there exists a need for a rapid, non-visual, reliable, sensitive, low false-alarm rate detection process. Such a detection process can be particularly useful in the examination of transport items, such as by non-limiting example envelopes, parcels, luggage, trucks, or shipping containers. Such a detection process could also be particularly useful in the detection of explosives, biological weapons, or other materials concealed in the natural environment, such as antipersonnel or other types of land mines, explosives concealed in vehicles, and additionally useful in the non-invasive examination of munitions for use in determining appropriate deactivation procedures.
Magnetic inspection techniques can be useful for detecting metallic articles, such as weapons, but non-metallic articles such as explosives or narcotics are not detectable by such methods. Furthermore, explosives and narcotics, as a general rule, do not have readily distinguishable ionizing photon (X-ray or gamma ray) absorption characteristics, unless examined using low energy photons. However, the use of low energy photons limits such examination to smaller objects and, thus, may prove difficult to distinguish contraband and normal items of commerce. Both magnetic and X-ray and ionizing photon methods may not provide sufficient functionality and/or reliability in the detection of many types of contraband.
Thermal neutron activation analysis (TNAA) is a conventional method for determining chemical composition that utilizes neutrons to interrogate both samples in the laboratory and in concealed spaces. In TNAA, a source, generally of relatively low energy neutrons, is used to bombard the sample object of interest. The nuclei of the atoms comprising the sample capture the bombarding neutrons and are rendered radioactive. When these newly radioactive isotopes undergo radioactive decay, they emit energetic photons (gamma rays) or particles characteristic of the newly radioactive nuclei. These emitted photons or particles are then detected using various means by which the energies of the photons and/or particles are measured and used to identify the elemental composition of the sample object. This approach is capable of identifying various elemental compositions, but its practical utility is largely confined to low volume, low throughput applications as in a laboratory environment due, in part, to the high rate of attenuation of the low energy neutrons. There is also a low interaction probability for thermal neutrons in low atomic number materials, so a very large number of neutrons are needed for identifying these. Furthermore, a very large number of the possible thermal neutron interactions result in the production of radioactive nuclei that have persistent radioactivity. The fundamental process thus requires that the sample be made radioactive and the induced activity of many common materials, particularly metals, persists well beyond the inspection period and thus can present a health hazard. Items in the environment near the sample may similarly become radioactive. Because the composition of the materials in the container or space being inspected is not known in advance, the total amount of induced radioactivity and how long the induced activity will be present is similarly unknown. The duration of the radioactivity of many benign inspected items may be quite long.
Furthermore, neutron activation analysis is not very efficient for detecting nitrogen or carbon, two of the major components of explosives and narcotics, because nitrogen or carbon each have small capture neutron cross-sections as compared to heavier elements and metals. If these light elements are to be activated with high enough probability to produce useful signals for identification, the incident neutron flux must be relatively high and thus capable of inducing significant radioactivity in other benign components of the objects of interest being inspected. The method is further very insensitive to oxygen, an important component of many explosives and narcotics. Many explosives detection methods rely only upon the detection of nitrogen, but the nitrogen to oxygen ratio may be a significant indication of the presence of explosives and/or narcotics in an object.
While the discussion above describes the use of thermal neutrons (typically with energies below about 0.025 eV, the most probable energy) for activation analysis, higher energy (fast) neutrons may also be used. High energy neutrons have the potential advantage that they can penetrate further and thus may interrogate deeper into the interior of larger containers. In fact, their usage may be required for the interrogation of very large samples. Their use, however, suffers from the problems with persistence of lingering radioactivity in the object of interest and may pose other problems when the large neutron flux interacts with metals or heavy elements in the vicinity as described for methods using thermal neutrons described above. Furthermore, the capture cross-section for fast neutrons is often lower than for low energy neutrons and thus require an even higher incident neutron flux for reliable detection with low false alarm rates. Therefore, the target must be irradiated with large numbers of neutrons in order to get usable neutron activation data. This brings with it problems of unintended high flux neutron scattering that, for safety, requires substantially greater shielding, and thus a larger equipment footprint or personnel stand-off distances. It should also be understood that techniques employing neutrons having energies between those of fast and thermal suffer from some of the limitations of both techniques.
Inelastic neutron scattering (INS) is the process in which neutrons interact with the nuclei in object of interest producing an excited nuclear state, which then emit neutrons with a lower energy and gamma ray photons. In inelastic scatter the neutron induces an excited state in the target nucleus, which then “falls back” to its earlier energy by emitting a lower energy neutron and a gamma ray photon. Both the gamma ray photons and the neutrons have characteristic energies that, if measured, can identify the nuclei from which they were emitted. Inelastic scatter interactions do not make the target radioactive. Although there is a “delay” between when the neutron first interacts with the nucleus and when the gamma ray is emitted, this time is on the order of micro or nanoseconds, after which the target nucleus is returned to its previous state. The target may still be somewhat radioactive because a few of the source neutrons would be absorbed (as mentioned earlier) but this would not be as much of an issue as for absorption methods because inelastic scatter has a much higher interaction probability which would make it possible to have a much lower incident flux. Whenever neutrons are incident upon an object, especially objects containing lower atomic number materials, neutron elastic scattering also occurs with a very high probability. Inelastic neutron scattering methods do not use any information about object composition that arise from these very probable events. Thus methods using inelastic neutron scattering alone would require more neutrons to obtain the desired information than methods which use elastic scattering or a combination of elastic scattering with other processing. The persistent radioactivity and radiation hazards in INS would thus be higher than for methods taking advantage of neutron elastic scattering.
Neutron transmission analysis (NTA) has also been proposed for the inspection of closed containers. This approach uses neutrons in a similar fashion to ionizing photons to produce a “shadow” (transmission) image of the contents of a container. This method is identical to the way medical X-ray imaging works, except neutrons are used instead of X-ray photons. Neutrons are directed towards the object of interest and the transmitted (uninteracted) neutrons are detected on the far side by appropriately position sensitive detectors. Because some elements interact with neutrons much more strongly than others, the detected neutrons on the far side of the inspected object can cast an image whose intensity indicates the interactions with the object's components. This absorption may not be sufficiently specific to allow composition identification. A serious difficulty arises, however, because the absorption in elements other than the component elements of explosives and narcotics can be large and interfere with the detection process for the elements of interest. The high neutron fluxes required for adequate signal to noise to permit low false alarm rates again may generate radioactivity, as in TNAA, and thus the same safety hazard issues exist.
Another possible method for using neutrons to interrogate different targets is neutron elastic scatter (NES). NES has the highest interaction probability of any of the interactions discussed herein. In NES the incident neutron interacts by scattering off the target nucleus in a perfect elastic scatter. In these “billiard ball” calculations, the scattered neutron energy depends only on the incident neutron energy, the mass of the target nucleus, and the angle of scatter. Because this is the most likely type of neutron interaction for most materials (especially lighter ones), a smaller incident neutron flux can be used. This means that induced target radioactivity is minimized and thus there is less danger to personnel. This is the basis for the current application.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.